The present invention relates generally to power transformers of the helical or disc wound type and particularly to an improved static plate and clamping ring device for such windings.
It is well known that highly inductive windings, such as iron core transformer windings, when exposed to steep wave front or surge voltages, initially exhibit an exponential distribution of voltage drop along the length of the winding with a very high voltage gradient across the first few turns adjacent the line terminal or high voltage end of the winding. This non-uniform distribution of surge voltages is undesirable, as it necessitates thicker insulation at the high voltage end than at the low voltage end of such a winding. Size and cost of electrical apparatus are thus adversely affected. One well known and fairly standard partial solution to this problem, is the placement of a potential distributing static plate adjacent the high voltage end of a winding, with said static plate being electrically connected to a high voltage winding terminal. With such an arrangement, voltages applied to a winding of this type are capacitively coupled to the turns at the high voltage end of the winding which has the effect of increasing winding series capacitance, resulting in an improved distribution of such applied voltages. The static plate normally consists of paper tape having an aluminum foil backing, said foil backed paper tape being wrapped in a generally radial direction on a foundation ring of fiberboard or other electrical non-conducting material, said foundation ring being toroidal in shape in that it is in the form of a flat washer. The static plate, including its foundation ring, is placed adjacent the high voltage end of a winding of the above-mentioned type, and the aluminum or metal backing portion of the static plate is electrically connected to a high voltage winding terminal.
In addition to the above-mentioned static plate, it is necessary to provide means for clamping windings, of the above-mentioned type, along their longitudinal axes to control winding damaging axial movement of same. One technique is to place a plurality of insulated steel plates, sometimes referred to as quadrant plates, circumferentially around and adjacent the upper static plate surface or that surface of the static plate that is furtherest away from the high voltage end of the winding on which said static plate has been placed, said quadrant plates being, essentially, equally spaced-apart around said upper surface of said static plate. After the quadrant plates are in place, a jackscrew or other such device, reacted against fixed structure, is used to apply a force to the quadrant plate which, in turn, applies a compressive force to the adjacent static plate and associated windings. For mechanical stability and for economic reasons a single set of quadrant plates is used to simultaneously clamp high voltage, low voltage and other types of windings. Other clamping plates are in the form of semi-circles which necessitate the use of two such plates per phase winding. Using such plates to simultaneously clamp many types of windings, makes it essential that they be sufficiently insulated to withstand the highest voltage that any of these windings might be subjected to. In addition, each one of the quadrant plates, for example, must be separately positioned prior to being clamped in place, which is normally a time consuming task.
The use of a fiberboard or a permawood type material for the static plate foundation ring and the use of quadrant or semi-circular plates for clamping the end of a transformer winding necessitates the use of more material and space than would otherwise be used if a combined structural member provided these functions. In addition, if such a structural member was ring shaped it would provide the radial stability that is lacking in a quadrant or semi-circular plate arrangement. A ring-shaped structure would make possible the application of unequal compressive clamping forces to the high voltage low voltage and other windings, and would make it unnecessary to insulate all such clamping devices in a transformer for the same voltage withstand capability.